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Science: Occult events shed light on Saturn

By NIGEL HENBEST

A RARE astronomical event, which occurred last week, is providing astronomers
with new information about Saturn, its rings and its large moon, Titan.
On the morning of 3 July, Saturn and Titan passed directly in front of a
moderately bright star. The changes in the star’s light as it was hidden,
or ‘occulted’, are providing astronomers with new clues to the atmospheres
of the two bodies and about Saturn’s rings.

Until now, most of our information about Saturn has come from the two
Voyager spacecraft which flew past the planet in 1980 and 1981. Their close-up
pictures showed that Saturn’s rings are composed of thousands of narrow
‘ringlets’ and that Titan – the second largest moon in the Solar System
– is entirely covered by orange clouds.

The star that Saturn occulted last week, 28 Sagittarii, is a red giant
star 500 light years away. It is just visible to the naked eye, and is the
brightest star that Saturn has occulted this century. With electronic detectors
to measure accurately the star’s light and infrared emission, astronomers
had their best view ever of an occultation of a star by Saturn.

The event was visible from North America and westwards to New Zealand.
Carolyn Porco, of the University of Arizona, in Tucson, coordinated astronomers
at six observatories, in Arizona, Hawaii and Chile. They observed the event
at infrared wavelengths of 2 to 3 micrometres. In the infrared, a red giant
star appears much brighter, while Saturn appears dimmer. This makes it easier
to observe the dimming of the star.

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‘We were really blessed,’ Porco says. ‘It’s remarkable that all these
sites were free of cloud, especially as it is winter in Chile and the start
of the rainy season in Tucson.’ The observatories have provided thousands
of infrared pictures of the event.

When the star disappeared behind Saturn’s atmosphere, it blinked on
and off as it encountered layers of increased density, which focused the
star’s light. According to Porco, these lie in a region that the Voyagers
could not study in detail, dubbed the ‘ignore-o-sphere’. The spacecraft
provided information only on the uppermost layers and on the deepest layers
of Saturn’s atmosphere. The occultation is revealing structure in a middle
region of atmosphere, where pressures are between a millionth and a thousandth
of the Earth’s atmosphere at sea level.

As the star passed directly behind the centre of Saturn, the planet’s
atmosphere should have focused it into four images ‘dancing around the edge
of the planet’. But no one saw this effect, and Porco now believes that
Saturn’s rings prevented astronomers from seeing the images.

The star’s brightness also flickered rapidly when the rings occulted
it, as successive ringlets moved in front of the star. The observations
from the six observatories effectively give six different cuts through the
rings. Porco hopes to make out details as small as 5 kilometres in size,
which is as good as the best images from Voyager, and possibly as small
as 2 kilometres from some of the observations.

These infrared observations complement the results from Voyager, which
observed at ultraviolet, visible and radio wavelengths. They show particles
in the ringlets that are a few micrometres in size.

But the most important results will come from comparing the results
with the Voyager pictures. The spacecraft showed that some of the ringlets
are elliptical, rather than circular. Because Saturn’s rotation makes its
equator bulge outwards, the particles in these elliptical orbits feel a
gravitational force that makes the orbits gradually swing around. Porco
hopes to identify some of the elliptical rings that the Voyagers spotted
eight years ago. From their orientation now she will work out Saturn’s gravitational
field in detail. This will give clues to how matter is distributed within
the planet.

Several hours later, Saturn’s large moon Titan occulted 28 Sagittarii.
This event was visible from Europe and surrounding areas, including the
Canary Islands and Israel.

According to Andrew Hollis, observing from Cheshire, the star flashed
noticeably as it disappeared behind Titan, but less markedly as it reappeared.
Hollis and his colleague Richard Miles are amateur astronomers who have
equipped their telescopes with electronic detectors.

Astronomers in Britain saw Titan pass directly in front of the star,
and focus its light. Hollis and Miles saw this event as a flash of light
halfway through the occultation, even though they could not see the separate
images of the star around Titan’s small disc.

Bob Forrest at Hatfield Polytechnic in Hertfordshire also recorded these
events, as did Graham Appleby at the Royal Greenwich Observatory in Sussex.
The observatory’s large telescopes are now in the Canary Islands, so Appleby
used a ‘laser-ranging telescope’, a small instrument that is set up to receive
pulses of laser light reflected off satellites.

The gradual fading as Titan moved in front of the star, and the brightening
as it moved away, will show layers of enhanced density, and possibly layers
of haze, in its ‘ignore-o-sphere’. The central flash of light, focused by
much larger layers of the atmosphere, will reveal conditions much nearer
the surface. The combination of results from all over Europe will show how
Titan’s weather systems are distributed across this cloudy world.

Now astronomers have the considerable task of reducing the mountain
data from this chance event.